A SMT otherwise known as Smart Media Token is Steem's answer to Ethereums ERC20 tokens (slightly different, but similar concept), what this means is that anyone can launch their own tokens on the Steem blockchain. I would recommend reading the white-paper for a detailed explanation, but here's a quote which rounds it up well:
> Smart Media Tokens will give anyone the power to launch and sell Proof-of Brain tokens, which are tokens distributed by “upvote” and “like”-based algorithms and can be integrated with websites to align incentives and spur growth, while websites are empowered to adopt sustainable, currency-centric revenue models
How it will effect Steemit, I believe we will slowly see people shift away from Steemit, users will begin to use other sites which offer SMTs and they will become the tokens you earn. This will allow sites to decide how they want to reward to their uses and in term create their own value for the tokens of their site.
Steem will be the main trading pair for Smart media tokens. Steem power will be used to provide bandwidth to these tokens and will still be just as valuable. In terms of SBD, I'm not too sure, I imagine it will remain the same, offering a "stable coin" which can be traded for $1 worth of Steem.
Note: Some of this is my view rather than pure fact, although I believe that this is a fair view of what Steem will become.